This flag enables gcc to "see" the entire C file before making optimisationdecisions such as inline, which results in gcc making better decisions. Oneof the immediate effects of this is that static functions that are used onlyonce now get inlined.

gcc 3.4 has this flag as well, however gcc 3.x have a problem with inliningand stacks and as a result, enabling this flag there would cause excessiveand unacceptable stack use. This problem is fixed in the gcc 4.x series. The x86-64 architecture already enables this feature so it's well tested already.

-# Disable unit-at-a-time mode, it makes gcc use a lot more stack-# due to the lack of sharing of stacklots.-CFLAGS += $(call cc-option,-fno-unit-at-a-time)+# Disable unit-at-a-time mode on pre-gcc-4.0 compilers, it makes gcc use+# a lot more stack due to the lack of sharing of stacklots:+CFLAGS += $(shell if [ $(GCC_VERSION) -lt 0400 ] ; then echo $(call cc-option,-fno-unit-at-a-time); fi ;)